This invention relates to transformers having disk type windings with improved impulse voltage gradients similar to those disclosed within U.S. Pats. Nos. 2,279,028 and 3,387,243.
It is well known that highly inductive windings such as those employed in iron core transformers and reactors when exposed to steep wavefront impulse or transient voltages, initially exhibit an exponential distribution of voltages along the length of the windings with a very high voltage gradient at the first few turns. For example, approximately 60% of the voltage may appear across the first 5% of the turns of the winding at the high voltage end. This extremely nonuniform distribution of voltage is due primarily to the unavoidable distributed capacitance between each incremental part of the winding and adjacent grounded structure such as the core and the casing. Such ground capacitance is referred to as "parallel" capacitance when the low voltage end of the winding is grounded in the usual manner. The winding also inherently contains a distributed capacitance between the turns and between the winding sections. The effective sum of all the distributed capacitances between turns and sections associated with a particular disc winding arrangement results in a value of capacitance in series with the winding terminals. If this "series" capacitance alone were present, voltage distribution throughout the winding would be substantially uniform and linear. This would occur also if inductance alone were present. However, since both series and parallel distributed capacitances are inherent winding characteristics, the transient voltage distribution is a design consideration of importance.
U.S. Pat. Nos. 2,279,028 and 3,387,243 attempt to circumvent the nonuniform transient voltage distribution of a disk winding by supplying supplemental ground capacitance charging current which would otherwise flow through the series capacitance network of the disk winding.
Aforementioned U.S. Pat. No. 2,279,028 discloses a static plate arranged at the line end of the winding and electrically connected both to the line voltage lead and to a plurality of rib type external shields. The rib shields consist of a single turn each of electrically insulated wire arranged radially around the winding with the static plate located axially adjacent to the first winding section. The rib shields are arranged opposite the second winding section and extend axially along a portion of the remainder of the winding. When inner and outer rib shields are employed, the outer shields are electrically insulated from the inner shields so that the individual rib shields are electrostatically coupled rather than electrically connected. By providing the external rib shield network, a large portion of ground capacitance charging current flows through the external rib shields and not through the series capacitance network of the winding.
Aforementioned U.S. Pat. No. 3,387,243 utilizes a pair of upper and lower static plates to supply charging current which does not flow through the series capacitance of the disk winding and thereby improves the voltage gradient during impulse conditions. The upper static plate is arranged parallel to the turns that comprise the first disk winding section and only the first portion of the winding sections adjacent to the line voltage lead contain interleaved turns. The voltage gradient is substantially modified along the disk sections closest to the line end of the winding. The purpose of the lower static plate within the continuous portions of the winding sections is to decrease the large impulse transient voltage gradient which occurs between the interleaved disk sections near the line end of the winding and the continuous disk sections further along the winding. Although the voltage gradient under impulse conditions is improved by the static plates, they are not applicable to all disk winding configurations, and may simply transfer the large impulse transient voltage gradient problem from the area where the interleaved sections join the continuous disk sections to the area within the continuous disks just below the lower static plate.
The purpose of this invention is to provide a disk winding arrangement having a voltage gradient along the winding under impulse conditions which is nearly the same as the turns ratio voltage gradient.